US 7233818 B1 Abstract Magnetic resonance imaging method and apparatus are provided for mapping the internal or bulk motion of an object by labeling the phase of a specimen magnetization with a selected spatial function and measuring changes in the phase of the magnetization. The spatial function is selectable to provide magnetization phase modulation corresponding to displacements in a selected direction, such as a radial or azimuthal direction. Methods and apparatus for producing images based on magnetization phase modulation acquire image data based on stimulated echos and stimulated anti-echos. In an embodiment, a series of 180 degree pulses produces alternating stimulated and stimulated anti-echos that are measured and assigned to respective images.
Claims(30) 1. A magnetic resonance imaging method, comprising:
applying a first excitation radio-frequency (RF) pulse along a first excitation axis to produce a first transverse magnetization;
applying a first position encoding phase label to the first transverse magnetization;
storing a component of the position encoded first transverse magnetization as a first stored longitudinal magnetization by applying a first storing RF pulse along a first storing axis;
allowing a first mixing time interval of duration T
_{M }to elapse;after the first mixing time has elapsed, producing a transverse magnetization based on the first stored longitudinal magnetization;
decoding the transverse magnetization produced after the first mixing time;
generating a first image signal based on the decoded transverse magnetization produced after the first mixing time;
applying a second excitation RF pulse along a second excitation axis to produce a second transverse magnetization;
applying a second position encoding phase label to the second transverse magnetization;
storing a component of the position encoded second transverse magnetization as a second stored longitudinal magnetization by applying a second storing RF pulse along a second storing axis, wherein a difference between an angle from the first excitation axis to the first storing axis and an angle from the second excitation axis to the second storing axis is not an integer multiple of 180 degrees;
allowing a second mixing time of duration T
_{M }to elapse;after the second mixing time has elapsed, producing a transverse magnetization based on the second stored longitudinal magnetization;
decoding the transverse magnetization produced after the second mixing time;
generating a second image signal based on the decoded transverse magnetization produced after the second mixing time; and
combining the first image signal and the second image signal to reduce a contribution associated with a complex conjugate of a phase-labeled magnetization to obtain an artifact-reduced image of a specimen.
2. The method of
3. The method of
4. The method of
dividing the mixing time intervals of duration T
_{M }into a first interval of duration t_{1 }and a second interval of duration t_{2}, where the first and second intervals are selected based on at least one longitudinal decay time T_{1}; andapplying a 180° radio-frequency (RF) pulse to the specimen after the time interval t
_{1 }during acquisition of both the first image signal and the second image signal to reduce contributions associated with free induction decay.5. The method of
_{1 }and a second interval of duration t_{2 }such that t_{2}=ln [2/(1+exp(−T_{M}/T_{1}))], wherein T_{1 }is a longitudinal decay time and T_{M}=t_{1}+t_{2}.6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
dividing the mixing time intervals of duration T
_{M }into a first interval of duration t_{1 }and a second interval of duration t_{2}, wherein the first and second intervals are selected based on at least one longitudinal decay time T_{1}; andapplying a 180° radio-frequency (RF) pulse to the specimen after the time interval t
_{1 }during acquisition of both the first image signal and the second image signal to reduce contributions associated with free induction decay.11. The method of
_{1 }and a second interval of duration t_{2 }such that t_{2}=ln [2/(1+exp(−T_{M}/T_{1}))], wherein T_{1 }is a longitudinal decay time and T_{M}=t_{1}+t_{2}.12. The method of
13. A magnetic resonance imaging method, comprising:
applying a first excitation radio-frequency (RF) pulse along a first excitation axis to produce a first transverse magnetization;
applying a first position encoding phase label to the first transverse magnetization;
storing a component of the position encoded first transverse magnetization as a first stored longitudinal magnetization by applying a first storing RF pulse along a first storing axis;
allowing a first mixing time interval of duration T
_{M }to elapse;after the first mixing time has elapsed, producing a transverse magnetization based on the first stored longitudinal magnetization;
decoding the transverse magnetization produced after the first mixing time;
generating a first image signal based on the decoded transverse magnetization produced after the first mixing time;
applying a second excitation RF pulse along a second excitation axis to produce a second transverse magnetization;
applying a second position encoding phase label to the second transverse magnetization;
storing a component of the position encoded second transverse magnetization as a second stored longitudinal magnetization by applying a second storing RF pulse along a second storing axis;
allowing a second mixing time of duration T
_{M }to elapse;after the second mixing time has elapsed, producing a transverse magnetization based on the second stored longitudinal magnetization;
decoding the transverse magnetization produced after the second mixing time;
generating a second image signal based on the decoded transverse magnetization produced after the second mixing time;
applying a third excitation RF pulse along a third excitation axis to produce a third transverse magnetization;
applying a third position encoding phase label to the third transverse magnetization;
storing a component of the position encoded third transverse magnetization as a third stored longitudinal magnetization by applying a third storing RF pulse along a third storing axis, wherein an angle from the first excitation axis to the first storing axis, an angle from the second excitation axis to the second storing axis, and an angle from the third excitation axis to the third storing axis are different;
allowing a third mixing time interval of duration T
_{M }to elapse;after the third mixing time has elapsed, producing a transverse magnetization based on the third stored longitudinal magnetization;
decoding the transverse magnetization produced after the third mixing time;
generating a third image signal based on the decoded transverse magnetization produced after the third mixing time; and
combining the first, second, and third image signals to reduce either a contribution associated with a complex conjugate of a phase-labeled magnetization or free induction decay, or both to obtain an artifact-reduced image of a specimen.
14. The method of
15. The method of
dividing the mixing time intervals into a first interval of duration t
_{1 }and a second interval of duration t_{2}, wherein the first and second intervals are selected based on at least one longitudinal decay time T_{1}; andapplying a 180° radio-frequency (RF) pulse to the specimen after the time interval t
_{1 }during acquisition of the image signals to reduce contributions associated with free induction decay.16. The method of
_{1 }and the second interval of duration t_{2 }are selected such that t_{2}=T_{1 }ln [2/(1+exp(−T_{M}/T_{1}))], and T_{M}=t_{1}+t_{2}.17. The method of
18. The method of
19. The method of
20. The method of
dividing the mixing time intervals into a first interval of duration t
_{1 }and a second interval of duration t_{2}, wherein the first and second intervals are selected based on at least one longitudinal decay time T_{1}; andapplying a 180° radio-frequency (RF) pulse to the specimen after the time interval t
_{1 }during acquisition of the image signals to reduce contributions associated with free induction decay.21. The method of
_{1 }and the second interval of duration t_{2 }are selected such that t_{2}=T_{1 }ln [2/(1+exp(−T_{M}/T_{1}))], and T_{M}=t_{1}+t_{2}.22. The method of
23. A magnetic resonance imaging method, comprising:
applying an excitation radio-frequency (RF) pulse along an excitation axis to produce a transverse magnetization;
applying a position encoding phase label to the transverse magnetization;
storing a component of the position encoded transverse magnetization as a stored longitudinal magnetization by applying a storing RF pulse along a storing axis;
allowing a mixing time interval of duration T
_{M }to elapse;applying an RF pulse at a time point within the mixing time interval, wherein the time point is selected to reduce a signal contribution from free induction decay;
after the first mixing time has elapse, producing a transverse magnetization based on the stored longitudinal magnetization;
decoding the transverse magnetization produced after the mixing time;
generating an image signal of a specimen and producing an associated image of the specimen based on the decoded transverse magnetization produced after the mixing time.
24. The method of
25. The method of
dividing the mixing time interval into a first interval of duration t
_{1 }and a second interval of duration t_{2}, wherein the first and second intervals are selected based on at least one longitudinal decay time T_{1}; andapplying a 180° radio-frequency (RF) pulse to the specimen after the time interval t
_{1}.26. The method of
_{1 }and a second interval of duration t_{2 }such that t_{2}=ln [2/(1+exp(−T_{M}/T_{1}))], wherein T_{1 }is a longitudinal decay time and T_{M}=t_{1}+t_{2}.27. The method of
28. The method of
29. The method of
dividing the mixing time interval into a first interval of duration t
_{1 }and a second interval of duration t_{2}, wherein the first and second intervals are selected based on at least one longitudinal decay time T_{1}; andapplying a 180° radio-frequency (RF) pulse to the specimen after the time interval t
_{1}.30. The method of
_{1 }and a second interval of duration t_{2 }such that t_{2}=ln [2/(1+exp(−T_{M}/T_{1}))], wherein T_{1 }is a longitudinal decay time and T_{M}=t_{1}+t_{2}.Description This is a § 371 U.S. national stage of PCT/US00/21299, filed Aug. 4, 2000, which was published in English under PCT Article 21(2), and claims the benefit of U.S. application Ser. No. 60/147,314, filed Aug. 5, 1999, U.S. application Ser. No. 60/165,564, filed Nov. 15, 1999, and U.S. application Ser. No. 60/201,056, filed May 1, 2000. The invention pertains to methods and apparatus for magnetic resonance imaging. Magnetic resonance imaging (“MRI”) is a noninvasive imaging method widely used for medical diagnostics. To date, MRI methods for tracking the motion of an object over relatively long periods of time have been based on spatially modulating magnitude of the specimen magnetization according to a specific grid pattern, and observing the deformation of this grid pattern as motion occurs. In order to quantify the displacement vector of any small volume element (voxel), the positions of the grid lines and their intersection points are precisely defined. This usually requires human assistance, and precision is limited by image resolution or voxel size. The motion of voxels between grid lines cannot be measured directly, and interpolation methods are used to estimate motion. Other MRI methods measure voxel velocity by subjecting the transverse magnetization to a biphasic gradient pulse before readout, so that stationary spins do not accumulate a net phase change, while spins with nonzero velocity components along the gradient direction accumulate a phase change. By measuring such phase changes, one or more velocity components can be derived. While phase-contrast velocity mapping generally provides high spatial resolution and simple data processing, it is generally unsuitable for motion tracking, as it requires integration of velocity vectors from multiple measurements and mathematically tracking voxel positions. These integrations and voxel position tracking are difficult and prone to error. Internal and bulk motion of a specimen are mapped by labeling the phase of the specimen magnetization with a selected function of position at an initial time and measuring changes in the phase of the magnetization. Either or both of a longitudinal and a transverse component of specimen magnetization can be phase labeled based on the selected function. A phase labeled component of magnetization is stored by rotating the component to align with a longitudinal axis that is defined by an applied magnetic field. The time varying phase of the specimen magnetization is measured by producing stimulated echos or stimulated anti-echos, or both from the phase labeled magnetization. Measurements of the stimulated echos and the stimulated anti-echos are processed to produce respective images. The phase labeling function can provide a phase modulation based on displacement along any direction. For example, the selected function can be as a function of an azimuthal or other angle, so that rotational displacements produce phase shifts in the specimen magnetization. These and other features and advantages are described below with reference to the accompanying drawings. The MRI system A radio-frequency (RF) transmitter A specimen to be imaged is exposed to the axial magnetic field B With only the axial magnetic field B Application of a selected RF pulse can rotate a magnetization or one or more components thereof. An RF pulse of duration and magnitude sufficient to produce a 180 degree rotation is referred to as a 180 degree pulse and an RF pulse sufficient to produce a 90 degree rotation is referred to as a 90 degree pulse. In general, an RF pulse sufficient to produce a rotation α is referred to as an α pulse. The axis of rotation of such pulses can be selected based on the direction in which the corresponding pulse magnetic field is applied. Vector quantities are expressed herein in boldface. A transverse component M Phase Labeling In some specimens, some volume elements (“voxels”) are moving and experience a displacement between an initial time t The magnetization can be amplitude modulated, frequency modulated, or phase modulated. Phase modulation can be accomplished by modulating the magnetization M or a component of the magnetization M with a phase factor e In a Cartesian (x,y,z) coordinate system, the magnetization M includes a longitudinal component M To produce a transverse magnetization having a selected phase modulation according to a function f(r), the equilibrium magnetization M
If m(r) is small compared to the magnetization M Although phase labeling is described herein generally with respect to phase labeling with a single function f(r), multiple phase labels can be used to obtain a transverse magnetization M The longitudinal magnetization M Another method of producing phase-labeled terms m(r)e Generally the RF-gradient pulse combinations produce phase modulations of the form m(r)e Examples of Phase Labeling In a first example, the selected function of displacement is equal to a Cartesian component of voxel displacement. For example, if the x-component is selected, the phase difference is proportional to f(r(t The function f(r) can also be specified in cylindrical, spherical, or other coordinates. As a second example, voxels can be phase labeled based on a radial displacement r in cylindrical coordinates with a function f(r)=kr, wherein k is a constant. The series expansion described above can be used to determine an appropriate RF-gradient pulse combination to produce M The function f(r) can also be selected to be a function of the θ-coordinate (azimuthal angle) in a cylindrical coordinate system to label angular displacements, i.e., θ(t Mapping the Time-Evolution of a Phase Label The evolution of voxel phase after an initial phase labeling at time t Various exemplary acquisition methods are described herein that are suitable for measuring phase labeled transverse or longitudinal magnetizations (or both) that contain phase labels such as e As shown in This acquisition method can be further illustrated with the example of the phase-labeling function f(r)=kr, in cylindrical coordinates. After the decoding RF-gradient pulse combination that produces A(r) is applied, the terms in M Generally, by using an RF-gradient pulse combination A(r) to tip the magnetization M In the above example, k(r(t Data can also be acquired based on a phase-labeled longitudinal magnetization, such as M The longitudinal magnetization can be phase labeled to be M If the two terms in M Example Mapping of Phase Label Time Evolution Phase labels can be mapped as a function of time to track motion tracking over a period of time. At each time point after the initial phase labeling, a fraction of the longitudinal magnetization is tipped onto the transverse plane, and the resulting transverse magnetization is detected with any of the methods described herein. After data acquisition, the remaining transverse magnetization can be destroyed with gradient spoiler pulses, and this procedure repeated again. The process can be repeated until the phase-labeled longitudinal magnetization is expended. To ensure that only a fraction of the phase-labeled M For such motion tracking, the phase labeling is performed with a 90° RF pulse, a gradient pulse along the x direction, and a second 90° pulse. This creates the longitudinal magnetization M Example Mapping of Displacement with Phase Labeling A specific example of phase labeling is described with reference to cardiac functional imaging based on displacement encoding with stimulated echoes (“DENSE”). A phase labeling function f(r) is selected that is a dot product of r and a vector k such that the phase-labeling function is f(r)=k Referring to During STEAM imaging, all 90° pulses are applied along the same axis (for example along +y), during the first half The magnetization described by Equation 6 is missing phase imparted to the spins by the 90° pulses. For this description of STEAM, the second and third 90° pulses in the RF pulsing scheme (90° During the second half
Gradient pulses To describe phase label measurements, phases of two components of the signal magnetization can be written as ordered pairs. For example, the signal stored along the longitudinal axis after the second 90° pules, as described by Equations 5 and 6, can be expressed as
The fast spin echo (FSE) measurement shown in In a step A 180 degree pulse is applied in a step In a step Table 1 illustrates the phase shifts produced by the method of
While the echos are contaminated by S Simultaneous Dual-Echo Readout for STEAM The readout scheme presented above has the advantage of utilizing the full extent of the available magnetization for collecting data by sampling either the STE or the STAE at any given acquisition window. In addition, it requires no special post-processing tools when compared to existing slower versions of DENSE. However, this scheme lacks the ability of simultaneously sampling both components (STE and STAE) of the signal. By eliminating the second gradient encoding pulse (see Decoupling Overlapping STE and STAE in the Acquisition Window In both acquisition schemes described above, it is assumed that the two echoes (STE and STAE) are separated in k-space adequately by means of the encoding gradient moment, m With some encoding schemes, it is not always possible to utilize encoding pulses that will lead to such clear separation between the two components of the signal. A mechanism for distinguishing the two components in such cases is described below. For this description, it is assumed that the free induction decay (“FID”) has been suppressed. For example, with the STEAM pulse sequence, the application of the gradient encoding pulse, with moment m In phase-labeled imaging, phase-labeled components of magnetization decay or lose coherence via several processes that are conventionally characterized by time constants T In one example of phase labeling, the magnetizations of all voxels in a region of interest form spokes of a wheel in a plane perpendicular to the xy plane, after phase labeling as shown in At time t This process can be repeated for the series of time points: At each time t Each RF pulse is in this sequence, including the 90° pulses in the phase-labeling section, are fully balanced so as not to leave residual phase dispersion on the magnetization vectors, regardless of the initial orientation of the magnetization vector. A fully balanced slice-selective RF pulse In biological samples, usually T The RF pulses at the time points t This analysis leads to other embodiments in which the RF pulses for the time series alternate between 180° −α and −(180° −α), α being a small angle (e.g., 30°). Since each RF flip is about 180°, it refocuses most of the phase dispersions in M In other embodiments, one can replace the α-pulse train in Further embodiments include reading the transverse phase-encoded magnetization with a train of 180-degree pulses, right after the phase label has been applied. The signal sampled during the series of 180-degree pulses can be used to form images, which posses phase-labeled information at different time points. The length of this readout period will be limited by T There are many other possible embodiments of this general method in using a combination of other RF pulse series and readout schemes. If the RF pulses are slice selective, then the slice selective gradient needs to be fully balanced to avoid unwanted phase dispersion in the through-slice direction. This method is called the rotating-wheel method because the magnetization vectors form a vertical wheel in the spin space after phase labeling, and the wheel rotates around its axis during data acquisition (See A cycling between imaging and storing magnetization along the z-axis can be maintained by applying a series of gradient-balanced 90° RF pulses. In this series, the phase of the 90° RF pulses can be changed by 180° after every four such pulses. By doing so, for every eight RF pulses, any deviations from a true 90° rotation are compensated for. In addition, both halves of the magnetization spend equal time along the longitudinal axis and the transverse plane. As a result they decay according to the same decay-rate, which is approximately two times T The acquisition methods can be adapted for mapping phase labels at a series of time points to track motion. At each time point after the initial phase labeling, a fraction of the longitudinal magnetization is tipped onto the transverse plane, and data corresponding to the resulting transverse magnetization is acquired as described above. After data acquisition, the remaining transverse magnetization can be destroyed with gradient spoiler pulses, and this procedure repeated until the phase-labeled longitudinal magnetization is exhausted. To ensure that only a fraction of the phase-labeled longitudinal magnetization M Free Induction Decay Suppression Selection of τ Reduction of Phase Errors by Interleaved Data Acquisition Phase-labeled terms acquired during readout contain the phase-label function as well as other additional phase contributions from eddy currents, B Resolving Phase Ambiguities The phase of an MRI signal is normally expressed in the range of 0 to 2π radians. When a specified phase-label function exceeds this range, the acquired phase-label distribution contains step-like jumps of 2π magnitude. This phenomenon is called “phase wrap-around.” Phase wrap-around is corrected by first locating the discontinuous boundaries where this jump occurs, and then, for each boundary, the phase of the voxels on one side of the boundary is added or subtracted with an integer multiple of 2π, such that the discontinuity is removed. This procedure is generally effective. In some specimens, a bulk motion of an isolated region needs to be measured in relation to other regions and phase differences between these regions are ambiguous. In diagnostic imaging, the purpose of motion tracking is usually to characterize the internal movements of a contiguous area, where phase unwrapping is sufficient to resolve the ambiguity. In certain applications, measurements of local tissue deformation are needed, such as the strain in the myocardium. For these applications, it is not necessary to unwrap the phase for the entire region of interest as a whole, but rather it is sufficient to phase-unwrap each small area encompassing a group of neighboring voxels, and obtain the local deformation in this area. Strain Data Display In certain applications of MRI motion tracking, it is advantageous to quantify the deformation of a region by computing material strain. An example is strain mapping in the myocardium. In a two-dimensional (2D) plane, such as a 2D image through the long axis of the left ventricle, strain tensor maps can be calculated once the in-plane components of displacement vectors are mapped with one or more of the methods described in the previous sections. The strain tensor at each voxel is represented by the strain values (negative for compression and positive for stretching) along two orthogonal directions, called principal axes of strain. Both the strain values and the principal axes contain useful information in many cases. The strain values can be display using short, thick line segments of uniform length to represent the principal axes at each voxel, while the a color or a grayscale intensity of the line segments represent the strain values. The strain data can be presented in two strain images, each containing strain values of a particular sign, so that one map presents the axes and strain values for compression, while the other presents the axes and strain values for stretching. The color or grayscale intensity in each map represents the absolute value of the positive or negative strain. If a voxel has the same sign of strain for both principal axes, then in one map, two orthogonal line segments appear in its position, while in the other map the line segments are absent. Alternatively, the strain data can be separated into two maps containing the higher and lower strain values, respectively. Then, each voxel in a map contains one line segment, whose color or grayscale intensity represents the corresponding strain value. Since each map may contain both positive and negative strain values, the color scale or gray intensity scale may need to represent a range of values from negative to positive, and a mixture of color and gray-intensity scale can be used for this purpose. Strain data can also be displayed in a single image by providing each voxel with orthogonal line segments of uniform length to represent the principal axes of strain. The color or grayscale intensity of each line segment represents the corresponding strain value. A mixture of color scale and gray intensity scale can be used to cover a range of values including both negative and positive numbers. Example embodiments of the invention are described above. It will be appreciated that these embodiments can be modified in arrangement and detail without departing from the scope of the invention. Patent Citations
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